1. Field of the Invention
This invention relates to tangential blowers (also known as cross-flow blowers) generally and more particularly to tangential blowers used for circulating gas within a gas discharge laser.
2. Discussion of the Related Art
Tangential blowers have been used in various applications for several decades. Interest in tangential blowers has been heightened in recent years because such blowers are well suited for use in pulsed gas discharge lasers.
The operation of a typical electrical discharge system for a gas discharge laser is shown in FIG. 1. Electrodes 101 and 102 are separated by a gap where the gas discharge occurs. This discharge occurs quickly and is typically repeated many times per second. It is recognized herein that for various applications including microlithography, repetition rates of 1000 Hz and more may be used.
Laser gas is circulated around a chamber and through the discharge gap. The gas is circulated within the chamber by a cross-flow or tangential blower, as shown in FIG. 1. Cross-flow blower 103 comprises shaft 104, which is normally parallel to blades 105. The housing 106 contains the laser gas. When cross-flow blower 103 rotates, gas is circulated between electrodes 101 and 102. FIG. 2 illustrates the air flow through a cross-flow blower.
Operating in a gas discharge chamber places numerous demands on the blower. Normally, the laser gas is strongly electronegative and therefore corrosive. In addition, a pulse rate of 1000 Hz means that a blower may revolve at, e.g., 3,300 r.p.m. in order to clear the gas from the discharge region between pulses. At such speeds, the bearings, shaft and other structural elements are subjected to stresses and vibrations. At still it) further higher repetition rates such as 2 kHz or more, at which repetition rates it is recognized herein that it is desired to have excimer and molecular fluorine lasers capable of operating at, there is still greater demands on the gas flow speed. That is, to fully clear the gas through the discharge volume, or volume of space between the electrodes that participates in the discharge, from one pulse to a succeeding pulse, either the gas flow speed is to be increased or the electrode width is to be reduced, or a combination of these two, to ensure that the gas mixture clears the discharge region from pulse to pulse at these higher repetition rates.
In part because of the demands of operating in a gas discharge chamber, there have been recent attempts to strengthen tangential blowers and make their components more durable. One approach is described in U.S. Pat. No. 5,870,420, which is hereby incorporated by reference and which teaches the use of truss elements welded to the inside of the frame of a cross-flow blower, as illustrated in FIG. 3. This patent describes blades with a single radius of curvature. The blades are fitted into slots in the frame and welded to the frame.
However, the braced tangential blower described in the '420 patent increased the gas flow rate by only a few percent as compared to conventional tangential blowers operated at the same speed. In order to obtain this modest increase in flow rate, the trussed blower required an increase in current of 27% to 28% as compared to a non-trussed tangential blower. A blower which is stiffer than the trussed blower of the '420 patent would be desirable, so that the blower could be rotated faster without excessive vibrations.
In order to add stiffness to the blower and reduce vibration, the blower may be divided into two or more sections in an axial direction, as illustrated in FIG. 4. However, one consequence of dividing the blower into sections is that a region of non-homogenous laser gas flow is created in the discharge gap between the two electrodes. As shown in the cross-sectional view of FIG. 5, flange 201 divides the blower cavity into two axial compartments. The laser gas from both compartments is not allowed to interflow until after being discharged from the blowers and directly before entering the discharge gap between the electrodes. In the short distance between the end of the top portion of flange 201 and the discharge gap, the laser gas volumes from either side of the flange have not had an opportunity to properly interflow, and there is a volume of inhomogeneous gas as shown at FIG. 4 between the two laser gas volumes that are directed to the discharge gap. This inhomogeneous laser gas volume lowers the productivity of the laser. It would be desirable to provide a device for supporting the interior ends of the blowers without creating such a region of inhomogeneous laser gas.